Comparison of different approaches for estimation of creep crack initiation

In large components such as rotors defects due to manufacturing processes have to be taken into account and crack assessments based on findings of non-destructive evaluation are necessary. Approaches are used in remaining life estimations, for example:
• Time Dependent Failure Assessment Diagram (TDFAD),
• Two Criteria Diagram (2CD) and
• Nikbin–Smith–Webster-Model (NSW-Model).The TDFAD approach is currently being developed within the R5 procedures as an alternative to conventional methods for predicting incubation and the early stages of Creep Crack growth. A key requirement of TDFAD approaches is the evaluation of a time dependent creep toughness, denoted Kc mat. The 2CD approach has been developed independently in Germany to assess Creep crack incubation in ferritic steels. This approach uses crack tip and ligament damage parameters, RK and Rσ, respectively. Furthermore the NSW-Model is employed for the estimation of creep crack initiation by using the creep fracture mechanics parameter C∗. Calculations and used parameters were compared for a ferritic 1CrMoV-steel.